Men go bald primarily because of a hormone called DHT (dihydrotestosterone) that shrinks hair follicles until they can no longer produce visible hair. About 16% of men between 18 and 29 already show moderate to extensive hair loss, and that number climbs to 53% by the time men reach their 40s. The process is gradual, genetically driven, and far more predictable than most people realize.
How DHT Shrinks Your Hair Follicles
Testosterone circulates throughout every man’s body, but it isn’t the direct cause of balding. An enzyme called 5-alpha reductase converts testosterone into DHT, a more potent hormone. DHT then binds to receptors in hair follicles on specific parts of the scalp, particularly at the temples and the crown. Once it locks onto those receptors, it activates genes that trigger a process called miniaturization.
Miniaturization is the core of male pattern baldness. Each hair on your head goes through a growth cycle: a long growing phase (anagen), a brief transition, and a resting phase before the hair falls out and a new one starts. DHT compresses the growing phase, making it shorter with every cycle. The follicle doesn’t get enough time to produce a full-sized hair, so each generation of hair comes in thinner, shorter, and lighter than the last. Over many cycles, what was once a thick terminal hair becomes a fine, nearly invisible wisp. Eventually the follicle produces nothing visible at all.
The puzzling part is that DHT promotes hair growth elsewhere on the body, including the beard and chest. The difference comes down to how follicles in different locations respond to the same hormone. Scalp follicles in balding-prone areas have a unique sensitivity that body hair follicles lack.
Why Some Men Bald and Others Don’t
Every man produces DHT, yet not every man goes bald. The difference is genetic. The androgen receptor gene, which determines how sensitive your hair follicles are to DHT, sits on the X chromosome. Since men inherit their single X chromosome from their mother, this gene comes exclusively through the maternal line. Research published in the American Journal of Human Genetics found that genetic variation in this androgen receptor gene is the single largest factor in early-onset baldness, accounting for roughly 46% of the genetic risk.
This is why the old advice to “look at your mother’s father” has some truth to it. The average resemblance in hair loss patterns is greater between a man and his maternal grandfather than between a man and his own father. That said, the genetics are polygenic, meaning multiple genes on different chromosomes contribute. You can inherit risk factors from your father’s side too, just through different genetic pathways. The androgen receptor gene is the biggest piece of the puzzle, but it isn’t the only one.
The Typical Pattern of Loss
Male pattern baldness follows a recognizable sequence. It typically starts with recession at the temples, creating a slightly higher hairline that forms an M shape. For many men, this is the first visible change and can begin as early as the late teens or early twenties. The next stage involves noticeable thinning at the crown, the area at the top and back of the head.
Over time, the receding front and the thinning crown expand toward each other. In advanced stages, only a horseshoe-shaped band of hair remains around the sides and back of the head. These surviving hairs are genetically resistant to DHT, which is why they persist even in men with extensive baldness. Doctors use a classification system called the Norwood Scale to describe these stages, ranging from stage 1 (no significant loss) to stage 7 (only the horseshoe band remains). Not every man progresses through all stages. Some stabilize at an early stage and stay there for decades.
A less common variation skips the receding temples entirely. Instead, the entire front hairline moves backward uniformly without leaving a bald spot at the crown, progressing directly from front to back.
Other Factors That Accelerate Thinning
Genetics and hormones drive the main pattern, but other factors can speed things up or add a different type of hair loss on top. Severe physical or emotional stress can push large numbers of hair follicles into the resting phase simultaneously, causing noticeable shedding weeks or months after the stressful event. This type of loss, called telogen effluvium, is usually temporary but can make genetic thinning look worse than it would otherwise.
Nutritional deficiencies, particularly in iron and protein, can also contribute to thinning. Certain medications list hair loss as a side effect, and stopping or switching those medications often allows regrowth. These causes are distinct from male pattern baldness because they affect hair across the entire scalp rather than following the characteristic temple-and-crown pattern.
The Follicles Aren’t Dead
One of the most important findings in hair loss research is that balding areas still contain living stem cells. The follicle’s stem cells remain viable even after the follicle has stopped producing visible hair. What’s reduced is the population of progenitor cells, the next step down from stem cells, that actually build the hair shaft. In theory, reactivating those dormant stem cells could restart hair growth, which is the biological basis for most current treatments.
This distinction matters because it means balding is not simply follicle death. It’s follicle dormancy. The machinery is still there. It’s just been turned down so far that it produces nothing useful.
How Current Treatments Work
The two most established treatments for male pattern baldness both target the biological process described above, but from different angles.
The first approach blocks DHT production. A daily oral medication inhibits the 5-alpha reductase enzyme, cutting scalp DHT levels by roughly 64% and blood DHT levels by about 71%. With less DHT reaching the follicles, the miniaturization process slows or partially reverses. Many men see stabilization of their hair loss, and some experience moderate regrowth, particularly in the crown area. Results take several months to become visible because the hair growth cycle is slow.
The second approach uses a topical solution applied directly to the scalp that increases blood flow to hair follicles and appears to extend the growth phase of the hair cycle. It works independently of the DHT pathway, which is why some men use both treatments together. This topical treatment can produce visible thickening in thinning areas, though it requires ongoing daily use. Stopping either treatment allows the underlying process to resume.
Neither option reverses advanced baldness where follicles have been miniaturized for many years. They work best when started early, while follicles still have enough capacity to respond. For men with more advanced loss, hair transplantation relocates DHT-resistant follicles from the sides and back of the head to balding areas, where they continue to grow normally because their genetic programming travels with them.
Why It Follows That Specific Pattern
One question that puzzles most people is why the sides and back of the head keep their hair. The follicles in these regions have a different genetic makeup. They contain fewer androgen receptors and respond differently to DHT, making them essentially immune to miniaturization. This resistance is intrinsic to the follicle itself, not to its location on the scalp. When a surgeon moves one of these follicles to the top of the head during a transplant, it continues behaving as it did in its original location.
The temples and crown, by contrast, are densely populated with DHT-sensitive follicles. This is why balding almost always begins and concentrates in these areas. The boundary between resistant and sensitive follicles is what creates the familiar horseshoe pattern in advanced baldness, and why that particular band of hair can last a lifetime even when everything above it is gone.